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Why do we see many more 5th and 8th freedom cargo flights than passenger flights?
Usually, we can see many cargo flights operating between 2 foreign countries (and even as domestic flights within a foreign country in some cases). There are passenger flights that operate in the same way, but they are much fewer in number.
For example, this is an image of Fedex planes flying over Asia, and none of them originate/terminate at US:

But if you take any passenger airline, then it is very unlikely that they will have so many 5th and 8th freedom flights. Why do more cargo flights than passenger flights enjoy these freedoms?
So, what's happening here? Are cargo airlines awarded 5th and 8th freedom rights more easily, or do passenger carriers simply don't apply for them, frequently, or something else?

Since I had to look it up, here is what Freedoms of the Air mean. It should also be noted that in the referenced Wikipedia article, it states specifically that China and Russia were not a signatory to the International Air Services Transit Agreement that founded these "freedoms". And for obvious reasons airliners want to avoid overflights of war-torn areas like Pakistan and Afghanistan.

@RonBeyer yeah they want to avoid Afghanistan air space, but Pakistani air space is quite busy actually. Many Indian aircraft use it as well, as Indian Air Force has restricted the use of far western air space in India..

Let us skip Pakistani airspace, airlines request Centre

At least in the U.S., many of the cargo flights of East Asian carriers were/are second freedom flights, stopping in Alaska (or perhaps Hawaii) to refuel. Carrying only half the fuel and refueling midway makes the trip cheaper. Passengers don't like that, so it's mostly been abandoned for passenger flights, but cargo doesn't care.

@RonBeyer Looking at FlightRadar24, I can see dozens of aircraft over Afghanistan and Pakistan, from airlines including Ethiad, BA, Turkish Airlines and Lufthansa. It doesn't seem like anyone is particularly avoiding those regions.

I think the answer is simply that Cargo is far less picky about how it gets to its destination, as compared to People. A pallet can easily take three different flights to arrive at it's recipient, whereas most passengers wouldn't tolerate so many transfers.
Fifth freedom cargo flights are also often irregularly scheduled. For example, Atlanta Hartsfield-Jackson Airport receives the occasional cargo flight from Turkish Airlines and Singapore Airlines. It frequently receives flights from Korean Air Cargo and Cargolux, but these often originate at a variety of cities, such as Chicago, Dallas, or Anchorage.
In short, cargo is more flexible, which allows cargo airlines to be more flexible.

Due to the necessities of freight delivery, it is much more cost effective to arrange regular bulk shipments to distribution hubs for sorting before dispersing the packages out for delivery to their final destinations. Most of the major freight carriers have hubs located around the world.

In many cases these may distribute to smaller warehouses located inside individual countries before being resorted and placed on trucks or vans for delivery. This shuffling inevitably leads to numerous flights that depart from one country and land in another, neither of which is the original country of registry for the aircraft.


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What does the letter "H" on the upper side of ADANA word and mean?
As you can see on the STAR chart of Adana, there is a letter "H" with a black background for the VOR sign's line. I can't find the meaning.

Elsewhere on the chart, is a holding pattern depicted for that fix?

Customers have stated "We noticed an 'H' symbol next to an airspace
fix/navaid in the SID/STAR chart planview. Explain what this means?"
The "H" symbol serves as an indicator for crews to look for a
published holding pattern that is shown 'off flight track' in an
inset.

Jeppesen Web Site

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Is there a safe and legal way to use airplanes in laser tag?
Would there be a safe and legal way to use airplanes in laser tag? The pilot would not be shooting to avoid distractions; a passenger would do it. Could shooting an infrared laser tag gun at a plane cause any problems? Would it matter if planes shot at each other or people on the ground shot at planes? If it's safe, what's the lowest altitude at which the plane[s] could safely fly?
I read the question about paintball and its answers, but I think laser tag will be safe. There are no physical projectiles (other than photons), and since it's infrared, I don't think it will have the normal dangers of lasers, but I'm not entirely sure.
Also, this part isn't exactly about aviation, but would typical laser tag guns have enough range to shoot a plane from the ground or a target on the ground from a plane?

Laser tag guns will not have an effective range of anywhere near 500ft, the closest range you should even be thinking about. Also laser tag guns rely on activating sensors worn on the body, which they won't hit inside a solid object like a plane. Don't even think about this.

@DJClayworth I don't play laser tag, but I did some Googling and found laser tag guns with 400 foot range, so it's likely possible to make a gun with range to reach an airplane at 500 feet, and possibly higher. As for the sensors, the OP might be planning to make sensors to put on the outside of the plane? No idea how hard that would be or what the safety implications would be, but it would solve the problem of beams not reaching the aircraft occupants.

Probably not in regular ground-based laser tag. But there's a place in France where you can do (supervised) air-to-air laser tag. It's a bunch of former fighter pilots who got themselves two B&F FK 12, stationed somewhere around Bordeaux. Website (French only).

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Do airlines employ FAs from their destination countries?
I'm watching a documentary about daily business of an airport.
There was a crew briefing for a Lufthansa flight from Germany to India, and as there was an Indian FA, the briefing was held in English (i.e. he didn't speak German).
While it is advantageous to have an Hindi speaking FA on board of a flight to India, I wonder why he doesn't speak German.
To me, it seems unusual to have an employee living here, but not speaking our language. (Though this is possible.)
So, my question is:
Do airlines employ people in their destination countries, for example to have FAs speaking the language of the destination country? Or is there some kind of exchange project?

Thats was quite common in airline industry

Just a note: "indian" is not a language (there are some dozen major languages spoken in India).

@PaŭloEbermann, and I just edited that out and replaced it with Hindi, which is spoken by the majority of, but not all, Indians. English is also an official language there. (At least according to Wiki.)

The 747 carrier I retired out of had FAs based in New York and Tel Aviv, and were residents of their respective countries. The citizenship of the JFK FAs was not a factor insofar as I know as there were numerous green card holders.
Non-stop flights between JFK and Tel Aviv (and return, of course) were staffed by Israeli FAs. All other flights were staffed by JFK FAs. The primary reason for this arrangement was cultural. For JFK/TLV the back quarter or so of the aircraft was allocated to Hasidics to attempt to satisfy their preferences. For example, no movies were shown in the back because they were objectionable to the Hasidics.
The advantage of having Israeli FAs on the direct flights was that they knew how to handle the Hasidics. Scheduling problems would occasionally result in JFK FAs operating a JFK/TLV flight. It was not uncommon on such flights for the cabin crew to request a cockpit crew member to come back to help mediate a dispute. I never received such a request from an Israeli cabin crew.

Citizenship is not usually an issue, but a foreign national's right to work for a company outside of their home country can be. Case in point, in the US, a Permanent Resident Alien (aka green card holder) has the right to work in almost any job, with the exception of some very specific cases. However, if an FA did not hold either a GC nor citizenship, they would have to apply for an employment visa, most likely an H1B. Unless an exception exists that I'm not aware of, the petition for such a visa would be denied by the DoL, due to their minimum criteria for the issuance of work permits.

While valid points, @habu, it doesn't address the OPs question about a Lufthansa flight from Germany to India. I'm pretty sure they weren't worried about US green cards or H1B visas. ;)

@steveverrill No, I did not say that the pax demanded that a cockpit crew member leave the cockpit. What I did say was that the cabin crew asked for help from a member of the cockpit crew. Usually the flight engineer was sent back. Troublesome Hasidics would listen to a male with stripes on his shoulders, whereas they would often ignore female JFK FAs that didn't know how to handle them. The Israeli gals knew how to handle them. This was pre-9/11, by the way, and it was common practice for a cockpit crew member to leave the cockpit to stretch their legs. 3 man crew, so 2 still in the cockpit.

@FreeMan - you are correct in that Lufthansa probably doesn't care much about US immigration law and statuses when making their hiring decisions, but while the setting which led to the OP's post was fairly specific, his ultimate question seemed to be a bit more general in nature, so I wanted to highlight additional factors that can make answering such a question more complex than might initially appear.

Airlines do employ people in their destination countries for a variety of tasks, both customer-oriented (such as gate agents and baggage handlers), as well as administrative and/or operational (such as station managers, maintenance staff, etc.).
Now, with respect to flight and cabin crew, things can get a bit more complex. Generally speaking, crew are hired and certificated in the country where the carrier is based out of (and, incidentally, certificated to perform air transportation operations for hire), and would have to follow local employment and, potentially, immigration legislation should they wish to hire foreign nationals; certain countries may make exceptions to these rules for airlines and the transportation industry, making it easier to hire staff with the necessary language skills, but this is by no way a given.
With respect to FAs, major carriers make an effort to staff their cabins with FAs that speak the local language when travelling to international destinations. The FA might not have spoken German, but in addition to English, he might have also spoken Hindi, or any number of regional languages spoken in India, thus allowing the crew a greater probability of having someone who can can converse with any passenger on the flight.

Yes they do.
My earlier flight this year on Hong Kong Airline from Hong Kong to Bangkok has FA who are Chinese and Thai. Cabin announcements were made in 3 languages, Chinese, English and Thai.

Yes, Swiss does it for their Tokyo destination.

At least can you tell which city pair or flight number?

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How is lift transferred from rotor to chassis (or weight, the other way around)?
I am trying to find out how a helicopter rotor's lift is transferred to the chassis. Equivalently, how is the weight of the helicopter transferred to the rotor blades?
While looking for this online, I find a whole array of pictures of the drivetrain. The force enters the main gearbox (MGB), but leaves the MGB with a spinning axle going to the rotor. First I thought that something happens in the swashplate, but that is not well designed for bearing large amounts of force. So my thought is that somewhere in the MGB there is a nice piece of a bearing.
There is a question like this one for airplane propellers: How does the load transfer from a prop to the airframe? The bearings shown there cannot be found in the MGB of a helicopter (or, at least, I could not find them). It seems to me that a helicopter is hanging from its MGB. Can someone help me out? Pictures would be really appreciated.

The force is transmitted through the mast bearing.  Gearbox diagrams often omit that because they're already so complicated.

The load path is rotor blade to hub, to mast, to mast bearing set inside transmission, to transmission housing, to transmission mounts/struts, to airframe.

You might want to add the "Jesus" nut. Called this because if it fails, all the occupants are left to say is "Jesus".

EDIT: it is the  main gearbox of an H145 aka EC145 aka BK117

I might have found the right picture:

This cutaway displays the main gearbox of a H145 and nicely depicts all the bearings (in light blue) which are found in a typical helicopter gearbox. The one of interest is the one I circled in red: it is a double-row spherical roller bearing.
This particular type of bearing is typically used for rotating shafts. It tolerates slightly misalignment between shaft and bore and it can transfer high radial and axial loads. Just perfect for a helicopter shaft!
The bearing transfers the load from the shaft to the outer case of the gearbox (in green) which is in turn connected to the fuselage via the v-shaped beams (in yellow).


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Would shutting down the engine have prevented the Atlantic Southeast Airlines Flight 2311 crash?
In the crash of Atlantic Southeast Airlines Flight 2311, a worn propeller quill caused the propeller blades to go flat, which created enough drag on that side of the airplane to make the aircraft uncontrollable.
If this were to happen in flight, and the bad engine were completely shut down, would that reduce the drag enough to recover control?
Perhaps in the case of 2311, they might have been too low and too late, but in general, can this type of failure be recovered from? It seems that if the propeller were not being driven at high RPM by the engine that the drag might be less severe.

With the blade angle fixed at 3 deg, even with the engine torque at zero, it wouldn't help them.  Think of it as a very small gyrocopter's free spinning rotor descending in autorotation straight down (they can do that); it's making enough lift to slow the descent quite a bit (some autogyros with special landing gear can do vertical landings).  An engine with a prop fixed at 3 deg blade angle that is free to spin will spin madly in autorotation like a gyro going straight down, generating massive lift aft (instead of up, as in the case of the gyrocopter).
Having the blades stuck at that position, the only hope would be to have the blades forced to stop spinning and stay stationary, making the blades just act as speed brakes with an AOA, in our case, of 87 deg. But they have to be prevented from turning.  As soon as they start turning they will start making negative lift.
So, theoretically, if the airplane had a propeller brake (which some airplanes do; it allows the engine to be run at idle with the prop stationary) and it was engaged while the engine was shut down or at idle, the drag of a propeller that is braked and prevented from spinning is a fraction of an autorotating propeller (basically, the difference between a gyrocopter descending straight down with its rotor spinning and making lift, and a gyro with its rotor held stationary with the blades producing drag only and slowing the descent only slightly below falling object velocity).
A turboprop like that with blades flat and locked against rotating would probably be able to maintain control, and might even be able to maintain level flight (but probably not climb), as long as the blades are unable to start spinning and making negative lift.  So in the end, I'd say the only way to save that situation would have required two things: a prop that can be braked, and time to figure things out and engage the brake.

The description of the accident says the propeller went to a flat pitch of 3°, which is still positive. At such angle, the air is trying to spin the propeller in the same direction as the engine, so the engine running helps the propeller spin. Only if the propeller went all the way to negative pitch would the engine be the air trying to spin it in opposite direction and therefore produce additional drag, but it appears that propeller was not built to allow negative pitch.
So I don't think it would have helped. It probably wouldn't have made much difference in the very flat pitch.


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Can anyone help identify this old 1930s plane?
I would like to identify this plane, if anyone can help? All I can tell you is that it is from an old photo album and probably an English Airshow in the 1920's or 1930's.

That is the Boulton & Paul Overstrand. A twin-engine biplane medium bomber, built for the Royal Air Force of the United Kindom starting from 1933. The last one was already retired in 1941. Apparently this was the first bomber with a closed power-assisted gun turret.

Still building in 1941?  Seems unlikely.

quiet flyer - the Overstrands were built in 1933-34 and the last was retired in 1941. They were used as trainers in their last years.

You are both right! Corrected.

Thank you for your answer, bogl.


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What is aerodynamic imbalance?
What is aerodynamic imbalance on a propeller? The most amount of hint I got was the shape of the airfoil. All I know is that it is the different thrust each prop blade produces due to different angles on each blade station. I am looking for a more in detail explanation and some examples.

I think he wants to know that you have an understanding of P Factor, which is the main "imbalance" or thrust asymmetry issue that has a direct impact on you as the pilot when you fly the plane.


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Why is thrust available constant with speed for turbojet engines, when it varies with speed for turboprop engines?

Similarly, why is power available (nearly) constant with speed for a propeller engine, while it varies for a jet engine?

Turbojet thrust is far from constant. But it's real power curve is quite strange, having a local minimum usually around M0.2, then increasing to a maximum somewhere in the supersonic region (where depends on the specific engine) and then declines, because after all the available power is finite.

Turboprops and turbojets - or, more broadly, jets - produce thrust in somewhat different ways.
First of all, let's address the way thrust is produced. Per Newton's 2nd and 3rd laws, force equals acceleration times mass, and an action (accelerating the air) produces an opposite reaction. After canceling out the variables (the math is easy to find), thrust is proportional to T=v*m' (m'=mass flow rate), and power transferred to the air is proportional to P=v^2*m'/2. All velocities are  in the airplane's frame of reference.
Now let's go to how engines produce this thrust. A jet engine first decelerates the incoming air to a near-zero velocity, generating drag, then accelerates it to a constant velocity, higher than the initial one, producing thrust. Both v and m' for a jet engine vary across the envelope, but they change much slower than the plane's speed. The engine spends roughly the same amount of power per unit thrust at any velocity.
A propeller doesn't decelerate the air at all. It only accelerates the incoming air by some amount. This converts power P into velocity v. Now, per above, P~v^2. If you encounter air moving at 0 m/s, you need 5 kJ/kg to accelerate it to 100 m/s. If the air is already moving at 100 m/s, you need 15 kJ/kg to get it to 200 m/s.
The result is that propellers encounter less resistance at low airspeed, so they get more thrust per horsepower the slower the plane is. This allows them to push more air or push it faster, producing more thrust. For fixed-pitch propellers, this works by producing more air delta-V and less drag. High-performance turboprops tend to have variable pitch propellers, which will be adjusted to push more air (slower) at low airspeed, or less (but faster) at high airspeed.
Turbofans, combining a fixed-pitch ducted propeller with a turbojet core, are somewhat in between. They lose some efficiency and some thrust as they gain more speed, like propellers, but their curve is much smoother and closer to turbojets in this regard.
The above is an extreme simplification (as is the jet's thrust curve in the question), just enough to get the idea across. The jet's actual thrust is also non-linear, which has been addressed in another question: How (and why) does engine thrust changes with airspeed?

"A jet engine first decelerates the incoming air to a near-zero velocity" I don't recall having read this here (TBF, this is my aviation experience) before. Can you explain this bit some more, or is that fodder for a whole new question?

@FreeMan This applies in the airplane's frame of reference, where the air is moving and the plane is still. The compressor section of the jet engine decelerates the air in flight, or accelerates it on the ground, to the point that its velocity entering the combustion chamber is nearly equal in either case.

So it's not actually stationary with respect to the ground, but with respect to the engine?

@FreeMan Yes. In aviation, forces and velocities are normally considered relative to the aircraft.

This makes intuitive sense. Is there a formal proof for these models?

@Jimmy This is only a rough illustration of the principle! The actual math for jets is quite complex, because some of the energy lost to frontal drag is reused through pressure recovery is accounted for. The answer to a similar question has a bit more details.

This is not entirely accurate. Jet engines don’t only accelerate air. They accelerate fuel gasses (unlike prop engines), which have much more mass than the air, and the speed at which it is accelerated grows with the availability of oxygen. Not only that, but the incoming airspeed reduces the power needed to spin the compressor, so the engine becomes more efficient at higher speeds.

Assuming that the net thrust of a turbojet is constant is not correct. It is assumed to be constant (for simplicity by the aircraft performance engineers and usually valid for low subsonic speeds), but in reality, the performance is not constant, and it also varies with altitude. This is best shown by a simple simulation of a turbojet engine. The following graph shows the net thrust as function of the Mach number. It clearly shows that the net thrust is not constant with speed:

The inlet of the turbojet slows the flow down and creates the optimal conditions for producing the exhaust jet. A turboprop relies on adding energy to the freestream air, as the speed of the airframe increases, there is less energy can be added to accelerate the flow.

The lapse rate of jet engines is sufficiently small for a constant thrust model to be valid for back of envelope estimations.

A small nitpick: the axes on that graph do not intersect at (0, 0), which may mislead casual readers. In reality it shows less than 10% variation in the thrust output over the Mach range, but it looks larger when presented this way.

There are several effects which in combination make constant thrust a good approximation at subsonic speed.
Thrust is created by accelerating a working mass in opposite direction. Net thrust is the difference between the impulse of the air flowing towards the engine and the combined impulse of burnt fuel and the air exiting the engine (and propeller, if one is fitted), derived after the time. Since that impulse is the product of mass and speed, you can either accelerate a large mass by a small speed difference, like a propeller does, or a small mass by a large speed difference, like a turbojet does.
When flying faster, the entry impulse of a propeller quickly grows large relative to the exit impulse, so thrust goes down with the inverse of speed. On the other hand, the high exit speed of a turbojet results only in a small increase of the entry impulse relative to the exit impulse while speed increases.
But if that were all, even the thrust of a turbojet engine would drop when speed increases. But there is a second effect which helps to let thrust grow with speed. With the square of speed, to be precise. That is the ram effect which helps to precompress the air entering the engine. At subsonic speed, this just about compensates for the loss of thrust: At low speed, the growing entry impulse lets thrust drop a bit but at higher subsonic speed the ram effect becomes larger and raises thrust again, such that a constant thrust becomes a good approximation. However, at supersonic speed the ram effect becomes dominant and thrust grows with speed squared – until the absolute internal pressure becomes too high so the engine must be throttled (or the aircraft needs to fly higher) or the shock losses in the intake become too large and thrust drops again.


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Can anyone identify this biplane?
This biplane from the 1930s is from an English airshow. Can anyone identify it?

Now that we have established that it is a Gloster Gauntlet from the No 19 Sqn of the RAF, can we determine the airshow? My guess is the Empire Air Day of 1935, 1936, 1937 or maybe 1938. Location: RAF Duxford. @PhilS do you have any information regarding year or location?

Unfortunately I am unable to provide any further information as my father passed some time ago. In the 1930's he lived in Cambridge, which would make Duxford spot on! As fate would have it he served in the RAF from 1940-45. I really wish I had asked him questions about these pics when he was around.

Maybe it is one step before the Gladiator: The Gloster Gauntlet. Here the number of wing struts and the landing gear are a better match.

Gloster Gauntlet (picture source)
It must be noted that a lot of experimentation went on with those aircraft. The one in the picture above has guns added on both wings which made the aircraft too heavy and were removed later.
The type in your picture clearly has a different engine with a smaller and deeper NACA cowling - maybe it was used for testing the Bristol Mercury engine of the Gladiator.

To my eye, it looks like the unknown has more wing ahead and behind the struts, larger wheels on simpler-looking main gear legs.  There were a lot of rather similar radial-engine biplanes from the 1930s...

@ZeissIkon The wheels look indeed larger. With the struts I don't agree. Wing planform, dihedral and all that look very similar if not identical.

Of course, there is the different engine cowl, too.  The Gauntlet has almost a speed ring, while the question aircraft has a longer cowling.  In those days, that might have been a running change (aircraft built a year or two apart).

You are absolutely right Peter. I've changed the contrast of the picture of the unknown aircraft a bit, and the tail is clearly checkered and seems to have a K with four numbers below. This picuture shows the Gauntlets of the No 19 Sqn of the RAF, showing exactly the same colours as the picture of the unknown aircraft. It's a Gloster Gauntlet Mk1

@DeltaLima Wow, thank you! Excellent detective work! I noticed the numbers but did not do the work of making them legible.

Thank you for the excellent Detective work DeltaLima! As mentioned in the other comments, my father lived in Cambridge, so Duxford was right on the Doorstep!

Maybe the Gloster Gladiator but possibly not:

Main visual difference appears to be that the Gladiator has one pair of wing to wing vertical struts on each side, while the Question Image shows two pairs per wing.

also the landing gear has more struts in the question photo. But the Gladiator is close!


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How is blended winglet better?
*Make sure to read the following as I cannot explain my question in one sentence
Winglets are a kind of wingtip device that is installed on an aircraft to reduce drag and improve fuel efficiency. There are two types of practical winglets that have already been installed on real aircraft, the canted winglet and blended winglet.
Winglets work by reducing wingtip vortices while producing an aerodynamic lift vector that points inboard and also slightly forward. The slightly forward component, as many author states, is due to the change in flow direction because of the wingtip vortices at a high angle of attack. With the increase in AoA, the main-wing part increases lift with a high pressure field below, than the winglet part, thus producing another vortex around the transition part of the main-wing and winglet. At higher AoA, the vortex also increases and strikes the leading edge of the specially placed winglet, forming this slightly forward-pointing lift.
My question rises when speaking of canted winglet and blended winglet. For canted winglet, the cant angle cannot be decreased too much, or

A sharp interior angle in this region can interact with the boundary layer flow causing a drag inducing vortex, negating some of the benefit of the winglet.

from https://en.wikipedia.org/wiki/Wingtip_device
In the case of a blended winglet, the transition part of the main-wing and the winglet is smoothed. It is being said that this feature decreases the lift and gives a better aerodynamic load. However, this smooth transition, at higher AoA, should also decrease the vortex formed at the transition, as this feature also provides a smooth lift force transition. Is this what they meant by

This feature sacrifices some of the potential induced drag reduction, in return for less viscous drag and less need for tailoring the sections locally.

according to Boeing?
It seems counter-intuitive that blended winglets reduce more drag than canted winglet, as canted winglet would use up more vortex formed to produce a thrusting lift force. Speaking of this, then, how is blended winglet better?

The blended winglet has two advantages:

The smooth change of contours means that pressure gradients also are smooth and the flow remains attached through the operating range. The sharp corner of a canted winglet will provoke early transition in the corner, similarly to a poorly designed wing-fuselage intersection.
The smooth winglet has less surface area for the same winglet height. Instead of two straight parts with the length of the smoothing radius each, the length of the transition is only $\pi$/2 times radius.

"pressure gradients also are smooth and the flow remains attached", Any chances that the canted winglet can use the more intense vortex formed around the transition part to create more "vortex induced flow (inboard tilted flow)", that consequently produces greater lift on the winglet, also greater forward-pointing vector of the lift?

@Frank No, that vortex carries energy away. The winglet can only capture the incoming flow which comes at a sideslip near the wingtip.

What do you mean by "comes at a sideslip near the wingtip"? Isn't the sideslip flow triggered by the inboard circling vortices near the wingtip?

@Frank No, the lateral speed component is the result of low pressure on the wing. This attracts more air from all sides and causes upwash and, near the tips, lateral flow. The vortex is only the result of this flow component. You are confusing cause and effect, but you are not alone. A lot of people do. But when you think about it, the suction explanation makes a lot more sense.


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Where does "wasted" propeller power go?
Aircraft propellers are very efficient, with around 80-90% of input horsepower being applied to axial flow. However, that raises the question of where the remaining 10-20% of energy goes. I'm specifically wondering about the percentages that go into specific things, such as:

Noise (vibration of the air)
Heat (random vibration of the air)
Centrifugal flow (impulse applied orthogonal to the propeller axis)

On that last note, I'm curious about how much of centrifugal flow is "blade chasing" (following the movement direction of the blade that created it), and how much is instead span-wise along the blade
I know this is a rather strange sounding question, but any nonrandom motion is at least theoretical able to have some energy extracted. I know this is part of how contra-rotating propellers gain increased aerodynamic efficiency over traditional propeller systems, as the second propeller can utilize some of the centrifugal flow of the first to generate additional thrust. My theory is that, by mounting propellers close to the tip of a wing with top blade moving away from the center line, some of this centrifugal flow could be exploited to "pump" against span-wise flow. But even before the obvious practical concerns, this theory is dependent on there being enough unused energy in the system to run it without additional power load being required

Heat, as everything that goes wasted. This would be a nice question for physics.se

Some of it, yes, but I highly doubt a whole 20% of the applied power is generating heat

Could it be drag?  BTW heat would not apply to brake horsepower, only to engine efficiency delivering that horsepower from fuel burned, right?

@RileyRosell: why? There are not much options. Sound also go to heat (and I do not think it will consume too much energy, but you can estimate: think dB of an airplane and of an amplifiers, and the Watt of the amplifier). Some *wind" power, but also I do not think too much energy. I doubt it will break some molecules, radiation is possibly just from heat. Maybe some push backward (which requires so additional force, and so we should double count energy), but would it just create heat?

@RobertDiGiovanni: drag is finally heat too

How much power is converted to sound is negligible.  How much power you could then practically extract from that sound is even more negligible!

I'm not looking to extract energy from sound or heat. My plan is to take energy from the spiraling motion the propeller imparts on the air. That is, a propeller will, in addition to sending air backwards at increased speed, will also impart a tangential velocity in the direction of blade travel: at the part of the disk where the blade moves up, the Propwash should be diverted upwards. It should divert downwards on down movement, etc.

Btw the claim that "ContraRotating Propellers gain increased aerodynamic efficiency over traditional propeller systems" isn't correct. Two close contra-rotating propellers lose some 10 to 15% power due to aerodynamic inefficiency in respect to two isolated propellers (plus the mechanical complications).

Eventually, it all ends up as heat.  It always does.
Initially, there is some that is held in the swirl velocity of the propeller slipstream.  Over time, this velocity difference dissipates due to the friction within the air -- which generates heat.
There have been many studies on wingtip propulsors as a means of either increasing propulsion efficiency or decreasing induced drag.  It has many complicating factors, but it is something that others have studied from time to time.

Eventually, yes, but my question is more about immediately behind the propeller disk

@RileyRosell: it goes in aerodynamic drag which eventually get dissipated on heat due viscosity. Some power is also dissipated in the gearbox connecting the propeller to the engine: in this case the dissipation is due local structural deformations which eventually also transform in heat. Is this what you're looking for?

@RileyRosell I addressed this -- there is a swirl velocity.  This can be recovered to an extent.  People have studied placing propellers at the wingtip.  There is a measurable benefit to doing so.

Sorry about that, didn't mean to sound snippy


text:
Is it possible to calculate the distance to climb to 1000 Ft in Cessna 172S?
Is it possible to calculate the ground roll and distance to climb to 1000 ft in a Cessna 172S?
Why?  I live near a small airport with 2 active flight schools.  As part of a new noise abatement procedure the circuit path will be moving from 500 ft altitude to 1000 ft altitude with no turns below 1000 ft weather permitting.  I'd like to be able to calculate some typical scenarios that will result in determining approximately where the end of the departure leg and the start of the crosswind leg of the circuit will happen to understand what the net change will be.  If I can get the ground distances, I can use the scale line drawing tools on a Google Earth satellite image to draw in some eastward and westward ranges for where the crosswind leg might start, for example on a typical sunny early September morning like today.
I'm not a pilot.  I'm a retired computer systems techno-geek who doesn't mind plunking parameters into formulae to crank out numbers.  I'm just having trouble finding the formula or reference chart.
I've downloaded a Cessna 172S Pilot handbook and have the performance info.  I can get current weather conditions for the airport thru their AWOS or from aviationweather.gov.   I have a binder half full of reference material, sample pilot exam questions, definitions and terminology.  I have the current aerodrome chart for the airport.  I'm starting to muddle my way thru calculating the ground roll and distance to clear a 50 ft obstacle using that reference material.
What I can't find is anything that extrapolates that and figures out the distance along the ground to climb to what is essentially a 1000 ft obstacle.
Can anyone help?

From what I understand, what you want can be calculated using your ground speed and your climb rate (something around 700 fpm for a 172). But be aware that your climb rate will vary depending on a bunch of factors like temperature and pressure altitude. The same is true for your takeoff distance. It will change depending on airplane weight, engine performance, etc.

The POH published ground roll is 960 feet. (Performance Specifications, cover page)
The rate of climb from Sea Level is 730 Feet Per Minute, which is 82 seconds to climb to 1000 feet. (Section 5-18, Time, Fuel and Distance to Climb)
During that 82 seconds, the plane will have a airspeed of 74 kts, or 85 MPH, or 1.419 miles per minute.
So in 82 seconds, covering 1.419 miles per minute, the plane will cover 1.93 miles.
1.93 miles of climb, + 960 feet of ground roll = 11,200 feet, or 2.12 miles.

The last column of the table in Section 5-18 is "Distance in NM from Sea Level", which says that the distance it takes to climb from Sea Level to 1000 feet is 2 NM.
2 NM is 2.3 statute miles, which is pretty darn close to my calculation of 2.12 miles (difference of 950 feet).  So the answer cross-checks nicely.
From start of takeoff roll to 1000 feet MSL, you can expect about 2.1 - 2.3 miles, not counting headwind and temperature factors.

I re did the calculations, accounting for a 20 kt headwind. (I think that is the maximum that most Cessna students would take off in), and that shortened the distance to 1.5 miles.

With max headwind: 1.5 miles
  With no wind: 2.3 miles.

I think that covers your range pretty well.

If you are not near Sea Level, you can use a different row from the same table.   If you want, you can interpolate, or even integrate the climb between two different altitudes, but that would change the answer very little.

This is a good worst-case estimate. Since the airplane is climbing, it is following the path of the hypotenuse of a right triangle. The ground track will be slightly less. The ground track will also be less if there is any headwind.

I believe the result would be more accurate using the airplane's ground speed instead of airspeed. Since I assume the OP is about the distance on the ground (but I might be wrong). Great answer anyways!

@JimyPP, yes, but I have no way of knowing the ground speed since I don't know the headwind.   My calculation shows the factors that go into it, and the poster can incorporate headwind himself.

Thank you everyone.  If I were to incorporate temp & wind speed (& pressure?)  into the calculation, is there a formula for that?  I'd like to get at least 4 plot points: 30C, no wind; 30C, wind 11 knots NW 310 degrees;  10C, no wind; 10C, wind 11 knots NW 310 degrees.  That would help establish a reasonable range for the end of the departure leg accounting for climate.  Is it worth doing all those calculations or could I just I could use 2.12 miles + or - a percentage to account for temp and wind.  Any ideas on what that + or - percent might be?

I think temperature and pressure will have only a minor effect on distance.  Wind speed will be more significant, and the POH covers calculations for that in detail in sections 5-4 and 5-14.  (note: It is stupid to ask us for calculations based on wind at 310 degrees without also telling us what the runway heading is!)

I see the updates above and think the range abelenky provided is something I can work with.  My apologies for not providing you with all the info you needed.  Like I said, I'm not a pilot.  But I sincerely appreciate the effort you put into providing the answer.  I was prepared to crunch the numbers myself if I knew the set of formulas to use.  Again, thank you for your time and effort.

If you think engineering is simply plugging numbers into formulae and cranking it out, then I feel sorry for you.  Understanding the meaning behind the numbers is crucial.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Again, where is the pivot point in pitch
In regards to recent question about neutral point, as being defined as center of lifts from wing and tail, could one also define a "neutral point" for down forces CG AND down force (up elevator) from tail.
Lift force is now only from wing
Notice if the down force center is behind the lift center, the nose pitches up.  Would the center of this rotation be at the wing, at the center of all moments (between Clift and Cdown), or somewhere else?

If tthis question relates to another, it might be helpful to link to it.

Is this the question you were thinking of?  Is the aircraft neutral point a function of the tail incidence angle?

Note: I rolled back the invalid edit that completely changed the meaning of the question!

… and after reverting to the original subject, it seems like you already asked this in What is the pivot point of pitch change on an aircraft?.

Disclaimer:  I am no expert and if you get a better answer from one of the other contributors here, you should listen to them, not me.
From what I can tell, it looks like you're asking whether you can define a point that includes all downward forces on the airplane, including center of gravity and tail downforce.  As I understand it, yes you can, and this point is center of gravity, which includes all the associated downward forces.
The airplane then would pitch about the center of gravity, as it usually does.

Also don't forget about the thrust vector.  If the thrust vector is below the center of gravity as is the case with high wing airplanes, thrust produces an "up" force or a torque of positive pitch.  Pitch is determined by the relation between the moments of up forces (against gravity) and down forces (CG).

Thank you so much!  Now I feel so much better about the theory.  Including all down forces works.  Now my tricycle gear can rotate in peace when I pull back on the stick!

Centre of gravity is, as the name suggests, the action point of gravity alone. The downforce—if it exists, because there might as well be upforce—of the tail acts way behind centre of gravity (or ahead for a canard), so the action point of this sum is not the center of gravity. But that point is not particularly useful anyway.

Yes, up force will factor into lift torques (to create a new centre of pressure), down force creates a new centre of down.  Notice "tail" consists of elevator AND stabilizer (both aerodynamic).  Small elevator deflection will rotate nose until stabilizer, now deflected into airstream, stops it.  New pitch angle is trim!  Meanwhile back at the wing, the center of wing lift is now (adjusted) directly over CG and of equal force, and plane is in straight and level flight.  Next up, rolls!

@RobertDiGiovanni, tail does NOT consist of elevator AND stabiliser, it is a stabiliser with movable portion called the elevator. You can't talk about their lifts separately, because they are a single airfoil that changes camber.

Ok, see below.  I old retired researcher, so this is hobby. Thanks

Note: this answer was made to a version of the answer that was improperly edited and made little sense.

Forces do not converge at points.
You can take a set of forces—any set of forces—with different action points and determine the action point of their sum, which is the point in which all their moments cancel.
Since the force is sum of the constituent forces and the moment (around arbitrary fixed point) is the sum of the constituent moments, you can simply find the moment arm that satisfies the two conditions and that is the action point¹.
Now you can do this with any set of forces, but it may not be useful. In case of aircraft, it is useful to sum the lift forces—their action point is called centre of pressure. And compare it to gravity, action point of which is the centre of gravity.
Summing all down forces is much less useful though, because the set of down forces changes with angle of attack and elevator deflection. As the angle of attack increases, at some point the lift force of the horizontal stabiliser changes from downward to upward!
Note that this point is not the neutral point. The neutral point is the point around which the total moment of lift forces does not change with angle of attack. That however, does not mean that total moment is zero around this point. Quite contrary; for statically stable aircraft, the moment around neutral point is pitching up, and the center of gravity is ahead of the neutral point to balance.

¹ In the general form, the equations are $\vec F=\sum\vec F_i$ and $\vec F\times\vec r=\sum\vec F_i\times\vec r_i$. In the linear case where all arms are along one line (e.g. longitudinal) and all the forces are in the same parallel direction (e.g. vertical), you can solve that as $r = \frac{\sum F_ir_i}{\sum F_i}$, but there is no division operator corresponding to cross-product for the vector case.

Appreciate the response.  Someone edited the question title.  My focus is understanding points of rotation in an aerogravimetric environment, which I picture as a yin and yang of CG and aerodynamic forces around a point.  This is why summing up and down forces (Including gravity on center of mass) may create a point of rotation not at the CG.  My theory, putting it to the test in a public forum.  I value the opinions found here.

You do well to sum the lift forces as centre of pressure.  Why not sum the down forces as centre of down (includes CG). Think of it this way:  plane straight, level flight.  Send 20 passengers to rear. CG moves back, nose up. Deflect elevator producing 20 passengers of down force.  Centre of down moves back, nose goes up.  It works!  In both cases tail will drop until up force from H stab (opposing elevator) brings forces back into balance.

@RobertDiGiovanni, you can describe the situation using any point of rotation, you can just get different linear motion of that point. However, standard law of motion state that acceleration of the centre of gravity equals sum of forces divided by mass and rotation of the centre of gravity equals sum of moments divided by moment of inertia. If you pick any other point, it will make additional movement due to the rotation, complicating your equations.

Not any point.  It is a dance around total lift, and total down force, out of balance.  That is the point of rotation in reality.  Once back in balance, rotation stops.  Notice this is not a contradiction of CG theory, only adding to it.  Try it.  2 see saws, one up, one down.  Find their centers.  How will it turn if out of balance (not vertically aligned)?  Thanks and good day!

@RobertDiGiovanni, See, a see-saw is an object with a fixed pivot point. A fixed pivot point will absorb any force needed for it to stay put. A see-saw still rotates around its centre of mass according to the sum of moments and its centre of mass moves according to the sum of forces—but this includes the forces through the pivot, which is why it is easier to analyse it using the moments around the pivot, because at that coordinate system, the forces through pivot add no moments.

@RobertDiGiovanni, … and similarly it makes sense to make parts of the analysis for aircraft relative to the neutral point—but the real neutral point, that is the point around which the moment does not change with angle of attack. However because aircraft does not have any fixed pivot point that would absorb arbitrarily high forces to keep it in place, calculating its motion with respect to any other point than centre of mass will be only more complicated.

@RobertDiGiovanni, this is all just basic dynamics, really. It would be useful to pull out the math and demonstrate how the calculations can be done with any reference point, but come out simpler for centre of gravity, but I am a bit out of practice. Somebody over on https://physics.stackexchange.com/ will probably be able to explain it better.

You're not out of practice at all, and thanks so much for your time.  I submit "what holds it in place" is equilibrium point of all forces.  I am working towards modelling of spins.  Probably in another question, no? Thanks again enjoying ASE!


text:
Can you use a mode 3AC interrogator in EU airspace?
I'm investigating the possible application of mode 3AC interrogators in EU airspace.
My experience is unfortunately limited to North America, and I'm unfamiliar with Mode S systems / function.
All the information I've managed to find describes mode S trabsponders as "backwards compatible" but fail to go into detail.
Is the backwards compatibility seamless and simultaneous?, Able to reply to both mode S and mode 3 interrogators in real time? Or a toggle switch between modes, in which when mode S is selected mode 3 interrogations will be ignored?
Do European standards allow mode 3A/C interrogators to operate within their airspace? Could this cause interference with local mode S infrastructure?
Are there any other potential concerns with operating a mode 3 interrogator in EU airspace?
To clarify I am speaking from the perspective of a ground station. As in using a mode 3AC SSR on an airport in EU airspace.
Any information would be greatly appreciated, thank you.

What do you want to use a 3A/C mode interrogator for?

Why would one be interested in putting money in that old technology that causes an extraordinary amount of extra transmissions on an already busy radio frequency?

I'd love to, but I can't go into detail due to privacy concerns, sorry.

Partial answer:

Is the backwards compatibility seamless and simultaneous?

Yes.

Able to reply to both mode S and mode 3 interrogators in real time?

Yes.

Or a toggle switch between modes, in which when mode S is selected mode 3 interrogations will be ignored?

No - a Mode S transponder will respond to interrogations by a "pure" mode 3 interrogator. Mode S interrogators have a special interrogation format that allows them to interrogate "pure" mode 3 transponders without triggering replies from mode S transponders even though those transponders do support mode 3 interrogations.

Do European standards allow mode 3A/C interrogators to operate within their airspace?

Don't know.

Could this cause interference with local mode S infrastructure?

Yes, in principle, for the same reason (FRUIT) that mode S was introduced in the first place - "overcrowding" of the airwaves. It won't be catastrophic. There are already plenty of airborne mode 3 interrogators operating (TCAS and AWACS).

Are there any other potential concerns with operating a mode 3 interrogator in EU airspace?

Consider both radio communication regulations and aviation regulations.


text:
How does cabin pressure vary with respect to ambient pressure during descent?
I came across this statement in a book -
"During a descent, the cabin pressure in a pressurized aircraft usually climbs at as lesser rate than ambient pressure".
Could someone explain this statement in simple words?

There's a graph of possible cabin and outside pressure for a flight on this answer.  https://aviation.stackexchange.com/a/26345/3159

During descent, the rate of change of pressure inside the cabin of a pressurized aircraft is lower than outside (ambient).

Inside the aircraft, under active control the  cabin pressure increases from equivalent ~8000ft above sea level to ground level. In the meantime, the aircraft descends from cruising altitude which is typically much higher, also to ground level.
Assuming the pressurization and descent start and end roughly at roughly the same time, the cabin is necessarily pressurized slower than the increase of air pressure outside in absolute terms  (e.g. psi per minute) assuming a steady pressurization profile .
Corner cases arise when the airport is at an elevation above 8000ft or so since the cabin will be further depressurized during descent. So while the relation holds in absolute magnitude , they will be of opposite sign. (until there is an airport  24,000 ft or so....)

The pressurization system uses a controller that manages pressure changes for the least impact on the passenger.  This means avoiding sudden changes and keeping the rate of change as low as possible, generally between 100 and 300 fpm, and never more than 500 fpm (about the most most people will tolerate without complaint) while the airplane is going 2000-3000 fpm.
One important issue the controller is designed to avoid is the "pressure bump" right after liftoff.  This is from the system reacting to the ambient pressure drop of the initial climb (the input/output response loop is quite long) and there can be a bit of a response overshoot that pops peoples' ears, generating complaints.
To avoid this, the system will slowly pre-pressurize the cabin to a few hundred feet below the current elevation before takeoff, too slowly for most people to notice, to get the system settled into its regulation mode and have a bit of pressure margin that allows the system to smooth out its own reaction to the sudden drop in pressure during the initial climb.


text:
What is aerodynamic damping?
Could someone explain what aerodynamic damping exactly is? I'm not able to understand the whole concept. Pls try to keep the explanation simple.
I tried to understand the concept from the link below. Is aerodynamic damping the same as the dihedral effect, or is that a whole different story?
https://encyclopedia2.thefreedictionary.com/aerodynamic+damping

You may add your current state of research. What ressources have you read so far? What is unclear for you in this ressources ?

Ok, I have edited.

I suggest you read how it flies

I think I missed seeing this question.  An interesting side note is that in the special case where rate of curvature of flight path, yaw rate, pitch rate, are all constant, the "aerodynamic damping" of pitch rotation and yaw rotation can be also viewed as a change in local angle-of-attack (e.g. of horizontal stabilizer, vertical stabilizer) due to the free-stream relative wind curving to conform to the curvature of the flight path.

There are some other cases where this concept also applies, but if the aircraft is pitching or yawing in a way that is not synchronized to the curvature in the flight path, (for an extreme case consider the famous "Cobra" maneuver, or a flat spin) then it does not apply.

The free dictionary is correct: Damping results from rotation.
Since all rigid-body rotations of a flying aircraft will occur around its center of gravity (this is a consequence of the conservation of momentum), that rotation will cause a vertical movement of parts which have a distance to the axis of rotation. This vertical movement changes the local angle of attack in a way that a moment is created which opposes that rotation.
Let's use a roll movement for example. The sketch below should indicate how the local angle of attack changes: The aircraft rotates around its lengthwise axis and one wingtip moves up while the other moves down (red arc segment arrows).

The rolling speed $\omega_y$ caused by the aileron deflection $\xi$ causes a vertical movement $\omega_y\cdot y$ that grows with the distance y from the rolling axis. This vertical speed (green arrows) adds to the speed of the aircraft v$_\infty$ (cyan arrows) and results in an inclination of the local flow speed (red arrows). Since the ailerons are deflected, the local lift (blue arrows) should be different between left and right. But the change in local speed adds a lift increment which is of opposite strength and equalizes the lift change from the ailerons. This stops the aircraft from accelerating the roll movement.
If the aileron deflection is set to neutral, the rotation will now result in a lift difference between left and right, and since that lift difference acts on a lever arm y to the center of gravity, it causes a rolling moment. This rolling moment acts against the rolling motion, so the roll speed will quickly drop to zero. This is aerodynamic damping.
The lever arm y occurs twice: The local speed change grows linearly with y and the opposing moment, being the product of the lift change and y, also grows linearly with y. The result is a damping moment which is proportional to y squared.
The free dictionary article also mentions that damping depends on density. With lower density, the flight speed v$_\infty$ needs to grow so the dynamic pressure stays constant. With a greater v$_\infty$, the local speed change from rotation will proportionally be smaller, and so will be the damping moment. Since dynamic pressure is proportional to density and speed squared, damping goes down with the square root of density.
The sketch also illustrates another effect: Adverse yaw. Since the local lift in attached flow is approximately perpendicular to the local flow speed, the local lift points forward on the down moving wing and vice versa. This causes a yawing moment which will rotate the aircraft around its vertical axis unless opposed by a rudder deflection.
Pitch damping works the same: Just exchange the subscripts!

Nice explanation @Peter Kampf. Do you have similar drawings/rationales for pitch and yaw as well?

@Kolom: No, but they are quickly made.

One question - I read somewhere, that during a turn we should even give opposite aileron, beacuse during a turn an upper wing moves faster than lower wing and thus upper wing generates greater lift force, and the result is overbanking tendency. And here, you write (correct me if I don't understand you properly) that at some roll angle without change in aileron deflection the roll rate becomes zero and if we move ailerons to neutral position the airplane will start rolling to level flight. What am I missing?

@Konrad What you know is correct. Most airplanes will fall into the roll when banked steeply enough - that is called roll instability. With moderate bank angles the aerodynamic and inertial uprighting tendency might be greater and bring the airplane back to level. But there are some which will enter a spiral dive when left to their own devices, even when flying level initially. There are several effects which influence the rolling tendency in parallel and the outcome depends on the individual design. But in general this happens slowly thanks to roll damping.

The link you provided has a poor choice of words: "restoring moment".
Damping forces are not restorative, i.e. they do not attempt to return the body to equilibrium, but rather oppose its movement. That is an important nuance.
Imagine a classic spring and dashpot vehicle suspension: the spring tries to restore the vehicle to its regular position, while the dashpot damps the velocity. By itself, the dampener will not restore equilibrium, it will stay at whatever position you extend it to, despite fighting the movement itself.
How does this apply to your question: aerodynamic damping is exactly that, the opposition  of the aircraft to rotation. You can observe it without flying: just imagine an aircraft hanging by its nose and tail, in such a manner that its CG is right between the supports. You can roll it by hand, but the drag on the wing will eventually stop this roll, but it will not level out the wings.
In flight, there are other forces at play that do provide a restoring moment, but aerodynamic damping does just what it says on the tin, it damps.

Technically, damping applies to translation too (for airplanes, mostly to up/down motion), not only to rotation.

Damping is a force that opposes motion, and is a function of velocity. It is an important factor in responses of equilibrium distortions - the classical way to explain this is to consider a spring/damper combination, as described here. Damping provides energy loss, so that the oscillation stops.
If an aeroplane is trimmed and stable, it can be flown hands-off in this equilibrium state. When now a wind gust blows the aeroplane nose up, the change in attitude will want to return the nose back to trimmed position. but like the spring system it will overshoot the trimmed position to the other side, return to trim etc.
Aerodynamic damping prevents oscillations from occurring indefinitely. It is damping provided by the aerodynamic resistance acting upon the oscillation movement of the aeroplane.


text:
Do FDRs & CVRs (black boxes) have an in-service life span?
Do Flight Data Recorders (FDRs) and Cockpit Voice Recorders (CVRs), aka black boxes, have an in-service life span? Is it necessary to replace them over time?
If so, are there any regulations governing their maximum life?
Note: The suggested duplicate asks specificly about post-crash lift time. This question is about in-service lifetime.
After a crash, does the information stored on flight recorders have expiration dates?

When asking about regulations you need to specify a jurisdiction such as FAA, EASA or other.

Black boxes?... Recorders is more accurate.

Possible duplicate of Do flight recorders have expiration dates?

I don't think it's quite a duplicate, although the titles are the same. The linked question is about how long the data last after a crash, not about maintenance schedules.

Yes, I am more interested in the maintenance schedules.

Considering that some aircraft have crashed with practically ancient recorders...

Modern FDRs and CVRs have a build in test equipment (BITE) that transmits any failure to the CMS (a system that collects the current failures). Having no moving parts these equipments do not require maintenance other than the scheduled pinger battery replacement.

The FAA is making an effort to phase out magnetic tape style recorders as noted in the this AC

1.7 Obsolete Technology. We are phasing out magnetic tapes through
  mandates and voluntary efforts and replacing them with modern
  solid-state recorders. These new recorders not only enhance safety but
  also benefit you directly, as they avoid the high costs and technical
  problems of maintaining outdated recorders. Modern recorders do not
  require the hourly replacement of tapes and calendar weighing of
  thermal capsules. Also, existing tape recorders no longer meet the
  most recent FAA TSO minimum performance standards.

However they clearly state the replacement is voluntary. The wording here also implies that the tapes are replaced on an hourly cycle and the thermal capsules are checked.
This AC covers quite a bit about the design and function of the FDR and CVR. I would take a look at CHAPTER 3. CONTINUED AIRWORTHINESS REQUIREMENTS starting on page 12. It outlines some of the things old FDR/CVR's may need to comply with to remain airworthy.
Im sure there are also specific limitations on specific units out there like any other component in aviation that may have caused them to be removed from service or lose their certification somehow.


text:
Why are there so few Lancasters still flying?
According to Wikipedia there are 54 airworthy spitfires in the world (30 in the UK alone), yet there are only 2 airworthy Lancasters (Lancaster B X in Canada & Lancaster B I in the RAF's Battle of Britain Memorial Flight).
As far as I'm aware they are similar aircraft (in terms of technology, obviously different size and role) from the same era so why is there such a difference in the numbers?
Is there a fundamental issue with preserving the Lancaster or is it simply a financial/lack of interest issue?

There may be some hard facts on this but I would say this question is largely opinion based.

@Dave as mentioned in the question, I wanted to know if there were any hard facts, such as not enough surviving airframes etc or if it's just the way it is because that's what people invested in.

Off the top of my head, it would just come down to the sheer size of the bombers. A Spitfire could be restored in a modest-sized barn by a single individual whereas a bomber would require a hangar and a team.

@Criggie - You might want to add the point about the relative numbers of people required.

Wikipedia claims 20.351 Spitfires and 7.377 Lancaster are built so I can expect lancasters to be more rare. Also The second was a Bomber and it means it's bigger, requires more maintenance, more crew and was usually in the wrong side of the guns

I can give you lots of reasons, but as with most things they all really boil down to one: MONEY.

Perhaps the first challenge with preserving any classic warbirds in airworthy "flying exhibit" condition is finding one, but let's assume you can locate a mostly-intact Avro Lancaster (in a field somewhere, or as a static display at a museum that hasn't done anything to it that would render it unflyable).
The next major hurdle is restoring it to airworthy condition economically:
A side effect of their role in the military is that these aircraft are flown hard (they have to accomplish their mission and a little airframe stress or engine damage in the process is OK as long as you still get it back home in mostly-serviceable condition for repair), and of course they do tend to get shot at (9 out of 10 structural engineers surveyed agreed that bullet holes are a Bad Thing for airworthiness; The 10th wet himself and ran out screaming). They've also been sitting around for decades suffering the effects of time (corrosion).
Once you find the aircraft you have to determine what repairs it will require to render the airframe airworthy. You will probably be replacing skin panels, control cables, hydraulic and pneumatic lines, etc. at the very least. The longer it's sat, and the worse the environment, the higher the chance of more major repairs (structural corrosion requiring ribs or spars to be replaced/spliced), and as these aircraft are out of production there are some parts you can't just get off the shelf: Custom fabrication for skins, control surfaces, ribs, etc. adds to the price tag.
Once the airframe is addressed you can look to the engines, since an aircraft without a powerplant is still just a static display.
Much like with the airframe you may find engine parts are hard to come by (the two airworthy B-29s, Doc and FiFi, both required a lot of effort to get working engines).
Now that you have a working engine you need to outfit the cockpit -- Many of the instruments the aircraft originally shipped with were likely pulled by the military or intervening owners as service spares for other flying aircraft, so what's left in the cockpit may be non-functional or so obsolete as to be useless. If you want an aircraft you can safely/legally fly to shows you'll probably need to invest tens-of-thousands of dollars in outfitting it with instruments and radios that are some functional combination of "authentic" and "modern".
Finally once it's all assembled, airworthy, and navigable you still have to pay to operate and maintain it like any other aircraft. When you're talking about a Lancaster with 4 fat engines on it you're racking up quite a fuel bill (and burning a good amount of oil with it).
The engines will of course need routine maintenance (oil changes, etc.) and eventually things will wear out or break, requiring repairs and leaving you with the "out-of-production parts" issues again.

Most of the "big" flying warbirds like the two airworthy B-29s and the two airworthy Lancasters are maintained by organizations that do it as a labor of love & out of respect for the history of these aircraft, but they do not have unlimited funding and there aren't a large number of near-airworthy airframes to choose from for these projects (with the pool of available hulks ever-dwindling), so in that respect there's certainly a financial factor as to why there aren't more of these flying.
Psychologically there's also probably something about having "at least two" airworthy examples flying that may make folks look at restoring a different type as well: Having two flying examples of aircraft from this period provides a decent level of insurance in so much as if one is lost in an accident or develops a problem that can't be fixed at least there's still the other one flying. (When FiFi was the only flying B-29 there were some folks who felt it shouldn't be flown, as an accident would cost the only airworthy example. Doc joining the flight line has muted some of those objections.)

The Commemorative Air Force put out a video on FiFi a couple years ago.  They mentioned that it costs roughly $10,000/flying hour to maintain....  Definitely not cheap.  Not sure how much a P-51D is.

There's a third Lanc being restored to flying condtion in the UK. They have got to the stage of funding the project by offering taxi rides along the runway at £300-£400 a head (and the approved emergency drill in case of brake failure is to take off, which says how close they are to completing the project). There is certainly interest, especially for events like both airworthy Lancs flying together in the UK in 2016. http://www.lincsaviation.co.uk/

+1 So from that I take it there are simply less airframes available than the Spitfire and because of the size, more parts are required which makes it significantly harder/more expensive, with less interest it's rather inevitable there's a big difference. Also I imagine a private investigator is more willing to spend money on a solo aircraft than one requiring a crew.

A number of Lancasters were converted to Lancastrians for civil use - and used until 1960. No doubt many were cannibalised to keep them flying, too. Same thing happened to Halifaxes. https://en.wikipedia.org/wiki/Avro_Lancastrian

Pretty much all the Lancasters built were either scrapped or used until their life expired - both the RAF and CWHM Lancasters have had significant structural parts replaced during their lives.

@Moo Aside from the 3,249 that were lost in action during World War II.

Another point that decreases the life of items constructed in wartime, is that raw materials are in particularly short supply and that one must make-do with what is on hand.

Consider that the British were offering up their metal garden railings for scrap to make armaments, bones for explosives, rubber and paper too.
This is one of the reasons the De Havilland Mosquito was make from wood and cloth, because both was available in better quantities than steel and aluminium.
An airframe thrown together quickly to plug a gap in the lines is not going to be built to last.   Metal parts that would have benefited from galvanising or wooden parts that should have been pressure-treated simply weren't done.
Consider the rated service life of a Wileys Jeep in the front lines was 30-50 hours (that's measuring the engine running time) or 90 days.  Its hardly worth changing the engine oil!
Size In a physically larger aircraft like a bomber, there's simply more parts to fail compared to a smaller and simpler fighter craft.
Storage A bomber is larger and will take up a lot more space for storage over 75 years.  A fighter with wings removed might fit in a large car garage, but a bomber would need a full-sized barn or proper hanger to be stored for minimum deterioration.

The storage is a very good point.

Most relevant faactors have been covered, but a summary may help newcomers.
While the service life may have been short (I recollect a figure of 42 hrs as the average service life of a Lancaster) this did not mean they were thrown together.  They were built to the best standards of the day, often better than today's GA aircraft, -BUT- The long-term behaviour of some of the materials used was simply not known. Any alloys containing magnesium present a corrosion problem - I've broken P-39 spars by hand when they have been exposed to the weather. So all-mag rivets must be replaced. Fatigue was also not understood, many alloys even in the Canberra were prone to cracking after a relatively short service life.
The standard of work required is the same as in a modern plane, so your workforce is likely to be predominately retired tradesmen. Training willing volunteers is necessary and time-consuming, on top of the time factor for the sheer quantity of work. Some of your workforce will probably die before completion!
The alternative is, as usual, money. American restorations seem to be able to fund full-time rebuilders amd engine overhauls. Price of a Merlin overhaul 20 yrs ago was $US110,000.
Operating costs can run away with any budget. 4 Merlins means about 200 gal/hr of 100/130, when you can get it, and 10 gal/hr of Aeroshell oil. And good luck sourcing the heavy grade of oil you need. Rule of thumb for operating cost is to multiply the direct cost of fuel, oil, insurance, etc. (you did think about insuring your crew, if not the a/c?) by a factor of at least 3 for a simple a/c, maybe 5 or more on a Lanc.
Then you need a crew, and where do you find a pilot current on a 4-engine tail-dragger with piston engines? Not to mention a flight engineer.
It just gets too hard for most, on top of the other factors of availability etc already covered;
Having been involved with warbird restorers and operators for about 30 yrs, including rebuilding a Spit XVI, I take my hat off to anyone with the commitment, and deep enough pockets, to tackle a large a/c project.
bleeding knucles.

This is just my personal opinion, but I think bomber aircraft have more ethical issues associated with them than fighter aircraft, and so are probably less popular. Spitfires are strongly associated with defensive actions like the Battle of Britain, so it's easy to see Spitfires as a good thing.
Explaining to a child at an airshow what a Lancaster was sometimes used for, such as the WWII bombing of Dresden is more difficult. "Bomber" Harris who was in charge of Lancaster forces remains a controversial figure.

The Longest Day (1962) and A Bridge Too Far (1977) featuring Spitfires each grossed about \$50m while the only significant film featuring the Lancaster, The Guns of Navarone (1961) only grossed about \$30m.
So we can estimate that the general public has had about 3x more exposure to the Spitfire than the Lancaster, perhaps explaining the disparity in interest.

Fighters are also sexier than bombers: Ask any kid who wants to join the air force if they want to fly the F-22 or the B-52 :)

You have touched on demand, but you forgot the corollary: cost.  Both have to be balanced to figure out if a venture is worth it.  Sure people know more about Spitfires than Lancaster, but Spitfires are also much cheaper to maintain.

Also consider that a moderately wealthy and motivated individual could actually fly a Spitfire, Mustang, or other WWII fighter craft, but would have to have a crew for a bomber.  I expect fighters are a lot more fun to fly, too, which adds to the motivation :-)

There are also replica Spitfires flying - but I don't expect anybody would even think of building an airworthy replica Lancaster.

"the only significant film featuring the Lancaster" - maybe "The Dambusters" (1955) is too old for some people to remember. Or maybe it's because it didn't involve the US military in any way ;)

@alephzero It may have been significant from an artistic perspective (I haven't seen it myself) but it only grossed 419k pounds in 1955, which is about $10m USD today. Whereas $50m in 1962 is about $400m today (incidentally, The Longest Day was the second-best-grossing film of 1962 after Lawrence of Arabia).

While we're drifting a bit here, in the UK /Dambusters/ had and has a massive cultural impact (not among film buffs). References to it are still everywhere, many of its themes are sung at football terraces, for example, and it's often used without need of introduction in advertisement spoofs, etc. Just the sort of thing to keep the Lancaster in popular imagination.

@DepressedDaniel the Dam busters film was borderline documentary IMO (yes I've seen the original black and white film, no I'm not that old I just like Aviation). I don't think documentaries generally do anywhere near as well as thrillers in box office ratings no matter how good they are.

Another factor to consider:
The Lanc is not alone in being produced in large numbers, but having few flyable examples. The US B24 was produced in great numbers, but there are only two flyable examples of a bonafide B24 today, plus a couple of navy variants that were used as water bombers until they were retired. The B17, on the other hand, has around 12 flying examples today, with more under active restoration.
Postwar, a flying Lanc wasn't all that useful in commercial aviation, although the design was morphed into the Lancastrian airliner and York freighter. Lack of pressurization was a factor in commercial aviation, as the pressurized Lockheed Constellation airliner could operate more efficiently with less weather interference at high altitude.
The UK doesn't have the huge forests that justify water bombers to fight fires, while the US and Canada do... quite a few US WW2 bombers ended up in that role, as they could be had for peanuts, and were kept flying into the 1970's.
The smaller B25 was found to be fairly useful in civil aviation as a transport aircraft, so many flyable examples of it still exist today, while the faster but more difficult to operate B26 didn't adapt well to civil aviation and the only flying example was crashed a few years ago. The B17 was also found to be more adaptable to postwar military and commercial uses than the B24, which is one reason that so many more examples of the B17 still exist. You may remember the B17 that was used to pick up Sean Connery at the end of the film Thunderball... they were still being operated commercially in 1966.
Consequently, there are fewer examples to restore, and a much smaller pool of spares to assist in the restoration. A primary reason so many B17's have been restored is the much larger pool of restorable examples, and especially the larger pool of spares that were preserved to keep postwar and commercial B17's flying.
In my garage, I have a crate of brand new electrical switch boxes that my father bought from an electronics warehouse in the late 1960's. It wasn't until I rode on the Collings Foundation B17, that I found out what those boxes were: they are the switch boxes for the intercom on a B17, one for each crew station. That's why they have a big fat paddle for the switch: to be used while wearing heavy gloves.
Same for P51's... kept operational into the 1980's by a few air forces, so a good pool of spare parts can be had. Also true of the F4U Corsair, more flyable examples of it exist than the more numerous F6F Hellcat, because the Corsair was kept operational into the 1950's as a ground attack aircraft. The Grummman Avenger torpedo bomber was also found to be useful as a crop duster and transport aircraft, so quite a few flyable examples exist today.
Spitfires were widely distributed to other nations after the war, which may account for the high number of flyable examples. Egypt kept a number of Spitfires operational in the 1950's, that clashed with the Israeli BF109's that Israel had concocted in a Czech factory. The Hurricane wasn't widely distributed postwar, so it, like the US P47, has few examples operating today.
WW2 aircraft use up spares at a frightening rate. If they aren't available, they have to be made by hand... very expensive.
Because the Lanc, like the B24, B26, and F6F wasn't efficient in a civil or military application post war, a large number of them and their storehouses of spares were scrapped quickly. The warbirds that fly in the greatest numbers today also happen to be those that were found to be useful postwar.

I know the Lancaster in Hamilton (Canada) is only flown occasionally (once or twice a year), due to the very high cost involved.  I've had the privilege of crawling through it in the hangar, and it's special for sure.  Although it's part of a fantastic museum of restored aircraft, there are never many people touring through (and generating revenue), very unfortunate.
I live about an hour away, and a couple of times over the years I've heard a deep rumble approaching in the sky, and instantly knew it was the Lanc out for a flight.  Can only imagine what a few squadrons of these together would have sounded like.


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What is this tab above the DC-10's #2 engine?
What is this big rectangular tab (?) thingy extending backwards just above the DC-10's #2 engine? What does it do?

(wikimedia.org, picture modified to point out the tab thingy)

(airliners.net via szextant.blogspot.com)
It mostly houses the same stuff that's on an underwing engine pylon: bleed air ducts, wiring, fuel and hydraulic hoses and tubes that connect to the engine core and accessories. The aft part of the structure where it tapers is just there out of aerodynamic necessity.

Also the stuff at the end of the fuselage below the engine is just a fairing. It will fold away and the engine is removed by installing a bracket on the pylon and using cables to lower the engine straight down. If it was deeper in the fin as Sean mentioned, it would need to be drawn backwards and then down, making it more complex to do an engine swap.

DC10 rear engine pylon, same as underwing, for supporting the engine. The tube that looks like it could provide support is just a very long intake duct.

So why not put the engine further forward in that tube, which would allow the engine to be supported by the horizontal stabilizer directly and allow its ancillaries to be placed within the stabilizer itself rather than needing an external pylon?

@sean You don't want an engine explosion to blow all the control surfaces off the aircraft. UA232's #2 engine failure took out all three hydraulic systems and control of the tail, but were able to land. If the engine had been mounted farther forward they might have lost the control surfaces altogether and consequently the plane.

@Pilothead: Good point, although it wouldn't matter nowadays (since all jet engines are now required to contain any and all debris within the engine under any and all possible failure scenarios).

@sean As they always have, and as well demonstrated by SWA1380, SWA3272 in 2016, QF032 in 2010, AF066 in 2017...

Ah, for some reason I thought that was a more recent (post-UA232) requirement...

You might want to add the bit about explodey-engine protection into your answer.


text:
What is a wing theory?
I have an assignment to summarize the topic "wing theory", with the preferred reference used is the book Fundamentals of Aerodynamics by John D. Anderson. But there's no dedicated chapter called "wing theory".
The gist I got is that the closest one is chapter 5, that's discussing "incompressible flow over finite wings". But I haven't got a clue what a "wing theory" is and what structure should I use to make a summary regarding that topic.
There are no search results on that topic either, not in Google, nor a Wikipedia page for it.
So, what is it really?

Lifting line theory maybe? Thin airfoil theory? Boundary layer theory? I'd ask your professor for a clarification

Well, I guess you'll have to "wing it".  But do check with the person giving the assignment.  Logically it flows from:  what wings are used for --> how do they work --> details in air flow --> how lift is created.  I would take a peek at Anderson's preface for insights on how and why the book was written.

Wing theory refers to the different mathematical tools which can be used to model the lift-generation mechanisms at work in a wing moving through (typically) air. These include:
Bernoulli theory, where the faster-moving air on the upper surface of the wing exerts a lower pressure on it compared to the slower-moving air flowing past its underside
Circulation theory, where the airflow surrounding the wing acts as if there was a net rotation of that air about the axis defined by the main spar
Vortex-sheet theory, in which lift is generated by the shedding of vortices at the trailing edge of the wing
No single model properly accounts for all the net lift generated by a wing in practice; each is a simplification, where some of the models furnish better predictions than others.

I would add to the excellent answer of @niels-nielsen theory of conformal transforms which greatly helps with airfoils - Zhukovsky transform,  Kármán–Trefftz transform etc

Wing theory is only a bigger term used to describe how lift is generated with regards to aerofoils and various shapes. Two utmost physicist and mathematician had described this generation of lift with their formulas. i.e the Bernoulli's theory and Newton's Laws 2nd and 3rd law
All in all the important thing to bare in mind is that lift can only be generated if there is a difference in pressure sorrounding the regions of a shape(or aerofoil for this matter such as the wing).
The word aerofoil is a generic term for objects that are capable of generating lift and this can be either a wing or a propeller. The wing theory can be described as:
In order for a wing to generate lift there as to be differential in pressure sorrounding the top and bottom chamber of the wing. The efficiency of the wing to produce lift is dependent on the transition point or the boundary layer(this is the point where the laminar flow of the relative wind breaks and becomes turbulent). The longer the laminar stream flows over the wing, the more efficient lift can be produced.
Another thing to consider is the wings aspect ratio. The higher the aspect ratio the more efficient the wing is to produce lift.
In addition you might want to also talk about the center of pressure on the wings. There's a lot of things involved in wing theory. However, the important thing is how you can describe each of the formulas(mentioned) and how they relate to lift and why aerofoil shapes are ideal.

"physicians" -- are you sure that's the term you wanted?

I meant physicist.

Just a couple of corrections if you want to implement them.

"lift can only be generated if there is a difference in pressure surrounding the regions of a shape": a symmetrical airfoil at zero AoA has plenty of difference in pressure on its surface but generates null lift.
"The word aerofoil can be either a wing or a propeller": an airfoil is a pure 2D concept so it cannot be a wing nor a blade which are 3D objects made up of several airfoils. 3. "The higher the aspect ratio the more efficient the wing is to produce lift": the higher the wingspan is, the lower the induced drag is.

Thanks for the correction. I've learnt a lot from your explanation

@LudwigT: you're welcome. The matter with the aspect ratio is unfortunately a deep rooted misconception, the wingspan is important, not the aspect ratio. And just for the record I didn't downvote your answer.


text:
Are there regulatory documents to carry medical kits and defibrillators on-board?
Are there any regulatory release documents applicable for carrying medical kits and defibrillators on-board a commercial airliner?
For example for:

Components, you are required to have an EASA Form 1 or FAA 8130-3 release tag.
Standard Parts, you can accept with a CoC (Certificate of Conformity/Conformance).

What are similar standards for emergency/medical equipment?

I think this depends on what type of operation you are. If you are going part 91, there isn't one. I also don't think there is one for commercial operations less than 10 or 12 passengers.

Thanks for the comment, it made me realise I should have clarified my question is for commercial airlines.

Again ‘commercial’ is a relative term and can apply to the Part 91 ops that Ron Breyer described as well.  You’re probably thinking of scheduled air carrier operations under Part 121.

Yes I'm talking about scheduled carriers such as BA, Lufthansa, Emirates, etc

You tagged both [faa-regulations] and [easa-regulations]. None of the 3 airlines you mentioned fly under Part 121 [faa-regulations]. Only BA and Lufthansa fly under [easa-regulations]. Could you clarify what country/region you want to know about?

Yes because all of these airlines maintain accept components with EASA and FAA release certificates. The question is whether any regulatory body from the two classifies such medical equipment in a category that requires a Form 1/8130-3 or CoC to be eligible for fitment.

These requirements will likely be found in the company manual, if not elswhere.

Thanks J Walter, I suppose from a regulatory perspective, there's no roadblock so the control mechanism would be the organisation's procedures!

I can’t comment on ICAO requirements, but for domestic and Flag Operations in the US, §121.801 - §121.805 and §121 Appendix A cover the requirements for emergency medical equipment, including electronic defibrillators, to be onboard scheduled air carrier aircraft and specify training requirements for flight crew members on their use.

Hi Carlo, indeed the requirement for having these onboard exists but I was wondering about any specific airworthiness or CoC document requirements? Or can such equipment and kits be carried without any such documents?


text:
Materials usually used in dorsals fins specifically in the ATR 72
I am an aerospace engineering student and I am having problems finding information about the materials usually used in the dorsal fin of an aircraft. I would also like to know the kind of stress and forces this part of the aircraft is under. The project I have to do is about replacing the material the selected part of the image is usually made of.
So my question is if anyone knows the material typically used in this part and if you may know any other alloy or material that could be used.

The loads on section of a dorsal fin are minor, just from the dynamic pressure of airflow that may be striking the panel from the side during yaw excursions in turbulence.  It's a "lifting" surface, insofar as it's contributing to the restorative yaw moment being part of a stabilizing fin, but the loads aren't super high and it may even just be attached like a fairing using dzus fasteners or camlocks or  similar if there is something inside that needs ready access.
Can't speak for the ATR specifically, but the material is probably either 2024 Alclad sheet like the adjacent structure, or if the part has compound curves, you might see a more formable alloy like 6061 or even 5052 in a non-structural part like that (you will see 6-series alloys in heated leading edges for example).  On newer designs it's more likely to be composite, usually Kevlar or a fibreglass/Kevlar blend (fibreglass is better in compression being stiffer, so it is sometimes mixed in with the Kevlar) and most recently carbon fiber.
If I was tasked with redesigning the part to a new material, it's likely because it's aluminum and the goal is weight saving, and I would likely choose carbon fiber as the replacement.  The optimum would be carbon fiber skins sandwiched over a nomex honeycomb core, the way interior floor panels are made.
An important caveat about using CF, is you cannot let it live right beside aluminum where there is a potential for direct contact. They are well apart on the galvanic scale and if there is water present the carbon will destroy the aluminum. If that's a problem it might be better to go with Kevlar composite.

This is a good general answer, I would go even further regarding Kevlar and it's properties under compression loads. Words terrible and suck come to mind

It seems the dorsal fairing is already made of composite materials. See the images below.
These components only receive aerodynamic loads and maybe unlikely yet possible foreing object impacts.

(taken from this paper)

(from google with "atr 72 tail fin fairing" keywords)
From the below  picture (from here) you can easily spot aluminium components (e.g. fuselage) in green from composite ones in white.


text:
Why are weapons pylon fittings usually tension fittings?
As far as I see, most of the fighter jet pylon fittings are tension fitting instead of the shear fitting. What is the logic behind that?

Might be because they need to be jettison-able.

Please define what you mean by tension vs shear. Wing pylons on the F-16, for instance, are held on with 3 bolts. 2 front one rear. They are NOT jettisonable.


text:
How can I best use the ACS?
I'm a student pilot and I feel I'm well studied but there are some concepts that I seem to forget. I'm trying to get beyond the point of relying fully on memory and more so using common sense to figure out some answers that may come up during my flight career. Is it normal to forget after you've studied?
Any tips on this?
I'm now using the ACS to study as I was just studying the Phak from cover to cover but what I'm realizing is that when looking at the ACS it's intimidating.
For example:

Knowledge
PA.IX.C.K1a A. partial or complete loss of power.

Am I to discuss what should happen if this was to occur or to describe why it could be happening?

The ACS is lists things the pilot should know, have adequate skill to perform and have an idea of the risks involved.
In fact, the ACS is separated into these three areas for each task involved.
The code you referenced is easy to figure out.
PA is private airplane ACS
IX is Emergency Procedures
C is Systems and Equipment Malfunctions
K1a is Engine roughness or overheat

The last section has a code of k, R or S
K is for knowledge areas
R is for risk areas
S is for skill areas

Listed under the task are the references for this task.
FAA-H-8083-2 - Risk Management Handbook
FAA-H-8083-3 - Airplane Flying Handbook
Airplane specific POH/AFM

The answer on how to deal with engine roughness will come from these references.  After you acquire the theory, you will have more common sense on the correct procedure to follow.
In this specific case, if there is engine roughness I would turn on the carb heat and pull out the checklist.  If there is no checklist, I would adjust the fuel mixture while contacting ATC and looking for an adequate airport to land at.  I would inform the passengers of the situation and the need to possibly divert to a different airport.  I would continually update my passengers with my plan of action as the situation continues.
I would think an answer from the examiner on why engine roughness would occur, how to possibly correct it and the decisions to be made on the flight afterwards would satisfy the ACS requirement.
The examiner will provide you a scenario during the oral and practical portions of the flight check.  They are looking for you to make the best decision available for the scenario while maintaining safety of flight and flying with the regulations
To be best prepared for the oral and flight check, I would suggest you read all the reference material you can.  Discuss different scenarios with your instructor both on the ground and in flight and just don't meet the minimum skill tolerances but be able to consistently fly with tighter tolerances.


text:
Does the VDGS have a sign for stopping too early?
I got a report from one our personnel who was watching the aircraft going in the stand under the Visual Docking Guidance System (VDGS). The aircraft was forwarding with extreme cautions and the captain kept her eye on the display that was showing distance remaining. At about the 1.5m display, the aircraft stayed on the spot for a second then VDGS display popped up the sign “STOP” despite the aircraft not having reaching the desired spot on the stand markings.
I complained about this situation to the VDGS operation team just showing "STOP" even though the aircraft was stopped too early. They said there is no problem about the situation, all things were in accordance with the procedure.
Is there really no problem about the way the operation team popped the STOP sign up even the aircraft was parked before the marking?
Does the VDGS system not have a separate sign for stopping too early? I searched on the internet and it seems like there is “TOO FAR” or “T-FAR” sign for stopping too late, but I cannot see another sign for too early.

I thought these systems are fully automated? There should be no-one operating them as such, there certainly is no "team" operating an individual VDGS. Plane type is somehow entered into VDGS, but then it should operate on its own.

It could be, @Jpe61, that the employee was visually monitoring as part of a testing/calibration, not monitoring as in "operating the system". Otherwise, fully agree with your question.

perhaps 1.5m is close enough? these systems are used with airliners, right? that are 30m long or so, or longer?

@user253751 Yes, they are used by airliners, but they usually have to park quite precisely. See here: Why do airliners have to park so accurately?